1. Field of the Invention
This invention relates to a radiation image recording and read-out method and system. This invention particularly relates to a radiation image recording and read-out method and system, wherein a residual image remaining on a stimulable phosphor sheet is removed in a radiation image recording and read-out system for iterately performing a radiation image recording operation for recording a radiation image of an object, a radiation image read-out operation, and an erasing operation on an identical stimulable phosphor sheet.
2. Description of the Related Art
Radiation image recording and reproducing systems utilizing stimulable phosphors have heretofore been known. With the radiation image recording and reproducing systems utilizing the stimulable phosphors, radiation carrying image information of an object is irradiated to a sheet containing a stimulable phosphor (hereinbelow referred to as the stimulable phosphor sheet), and a radiation image of the object is thus stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored on the stimulable phosphor sheet during the exposure of the stimulable phosphor sheet to the radiation. The light emitted by the stimulable phosphor sheet is photoelectrically detected, and an image signal representing the radiation image of the object is thereby acquired.
In the radiation image recording and reproducing systems described above, after the radiation image has been read out from the stimulable phosphor sheet, erasing light having wavelengths falling within a wavelength range of visible light is irradiated to the stimulable phosphor sheet, and residual radiation energy remaining on the stimulable phosphor sheet is thereby released. The thus erased stimulable phosphor sheet is again used for the recording of a radiation image.
In such cases, it often occurs that, on the erased stimulable phosphor sheet from which the residual radiation energy has been released, part of the residual radiation energy remains even further without being released perfectly during the erasing operation. The residual radiation energy remaining even further on the stimulable phosphor sheet without being released perfectly during the erasing operation is the energy due to the radiation image, which was recorded on the stimulable phosphor sheet in the manner described above. If the energy level of the residual radiation energy remaining even further on the stimulable phosphor sheet without being released perfectly during the erasing operation is higher than a negligible level, and the stimulable phosphor sheet, on which the energy level of the residual radiation energy is higher than the negligible level, is subjected to a next radiation image recording operation for the recording of a radiation image of an object, the residual radiation energy remaining on the stimulable phosphor sheet will cause a residual image to occur in a radiation image, which is reproduced from an image signal having been read out from the stimulable phosphor sheet having been subjected to the next radiation image recording operation. The residual image constitutes noise in the reproduced radiation image.
As a technique for rendering the residual image imperceptible, a technique utilizing offset processing has been proposed in, for example, Japanese Unexamined Patent Publication No. 6(1994)-014909. With the proposed technique utilizing the offset processing, the offset processing for setting values of image signal components of an image signal, which has been acquired from a stimulable phosphor sheet, to be large is performed on the image signal. In this manner, the contrast of a residual image represented by residual image components, which are constituted of the image signal components having comparatively small values, is set to be low, and the residual image is thereby rendered imperceptible in the image reproduced from the image signal.
However, with the proposed technique utilizing the offset processing, the values of the image signal components of the image signal are set to be large uniformly without discriminations being made between the image signal components, which represent the radiation image of an object, and the image signal components, which represent the residual image. Therefore, the problems occur in that, in cases where the image signal components of the image signal, which image signal components represent the radiation image of the object, are constituted of small values, the contrast of the image reproduced from the image signal becomes low, and the image quality of the radiation image of the object becomes bad.